One of the techniques that can be applied to packet communication in a 3GPP mobile communication system is the Robust Header Compression (RoHC) technique (see Non-Patent Literature 1). In the scheme of RoHC, the header compression efficiency at the packet transmitting end is adaptively varied (controlled) with the result of header decompression (restoring) at the packet receiving end.
A technique of RoHC is disclosed in Patent Literature 1 in which, upon detection of an error in the header of a received packet, the packet receiving end transmits request for updating context information to a packet transmitting end and when a number of requests for updating are received by the packet transmitting end in a predetermined period, the packet transmitting end transmits a packet having a header to be used for updating the context information to the packet receiving end after the packet receiving end restores the packet without referring to context information.
For example, the Patent Literatures 2 and 3 disclose methods in which: for example, an error in a packet having a restored header is detected and requirement of updating for stored context information is judged on the basis of the relationship between the number of packets having errors and the number of packets free from error; and a request for updating information is requested in order to update the context information.
Furthermore, the Patent Literature 4 discloses a method in which the number of ACK or NACK packets received in a unit time X is counted; when the operation mode of packet header compression is in the compression efficiency preference mode and the number of NACK packets exceeds the predetermined value Y, the operation mode is switched to the reliability preference mode; and when the operation mode is in the reliability preference mode and when the number of ACK packets received in the unit time X exceeds the predetermined value Z, the operation mode is switched to the compression efficiency preference mode.
In the Patent Literature 5 discloses, for example, a method of varying the operation of a compressor at periodic intervals, and varying the operation of the compressor in accordance with a feedback from a decompressor.    Non Patent Literature 1: 3GPP TS36.300 V8.1.0 (2007-06), 4.3.1, FIGS. 6-1, 6-2, 6.3.1, 3rd Generation Partnership Project, [retrieved on Dec. 3, 2007]    Patent Literature 1: Japanese Laid-Open Patent Publication No. 2002-94554    Patent Literature 2: Japanese Laid-Open Patent Publication No. 2004-229318    Patent Literature 3: Japanese Laid-Open Patent Publication No. 2004-215307    Patent Literature 4: Japanese Laid-Open Patent Publication No. 2002-135362    Patent Literature 5: Japanese National Publication of International Patent Application No. 2007-502073
However, the above conventional technique merely controls the RoHC state transition of each individual packet flows belonging to the same bearer (i.e., a logical connection) independently from one another.